1. Field of the Invention
Apparatuses and methods consistent with aspects of the present invention relate to a fixing unit for fixing an image on a print medium and an image forming apparatus having the same, and more particularly, to a fixing unit capable of adjusting the width of a fixing nip according to characteristics of a printing job and an image forming apparatus having the same.
2. Description of the Related Art
An image forming apparatus uses a developer (such as ink or a toner) to form a visual image on a print medium. An electrophotography-type image forming apparatus forms an electrostatic latent image due to an electric potential difference on an image carrying body, applies a developer to the latent image to form a visual image, and transfers the visual image to a print medium. To fix the transferred visual image on the print medium, the image forming apparatus includes a fixing unit.
The fixing unit includes a heating roller and a pressing roller, and performs a fixing operation by heat and pressure while the print medium with the visual image passes through a fixing nip formed between both rollers. However, the fixing nip is required to have characteristics which vary depending on the kind of printing job. For example, if the print medium is plain paper, the fixing nip is required to have a relatively larger pressure than that for special paper, such as an envelope where plural sheets of paper are folded. Further, if the fixing nip with a pressure for the plain paper is used in the fix of the special paper, the print medium may be creased or the like.
To prevent this, there has been proposed a configuration to adjust the width of the fixing nip by adjusting the distance between the axes of the heating roller and the pressing roller through a manual lever in the case of a printing job for the special paper, thereby controlling the pressure applied on the print medium passing through the fixing nip. However, this configuration is inconvenient for a user since he/she has to first manipulate the lever according to the kind of print medium.
Also, there has been proposed a configuration to control the speed of the print medium transported via the fixing nip according to the kinds of print medium. However, this configuration is not suitable since the transport speed of the print medium has to be controlled during the printing job, and is also disadvantageous to high-speed printing.